Method and system for handoff between an asynchronous CDMA base station and a synchronous CDMA base station

ABSTRACT

A method and system that enables faster acquisition of the forward link signal of a target base station in a mixed network of synchronous and asynchronous base stations is disclosed. The serving base station transmits in a neighbor list an estimated timing error  417  between the serving base station and a target base station. By utilizing the timing information, a mobile station estimates the relative time offset  408  between forward link signals received from the serving base station and signals received from the target base station. Timing information acquired during handoff enables accurate updating of the estimated timing error  417  subsequently transmitted in the neighbor lists by the base stations.

CROSS REFERENCE

[0001] This application is a continuation of co-pending U.S. patentapplication Ser. No. 09/515,042, entitled “Method and System for HandoffBetween an Asynchronous CDMA Base Station and a Synchronous CDMA BaseStation,” filed Feb. 25, 2000.

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] The present invention relates to wireless communication systems.More particularly, the present invention relates to a novel and improvedmethod and apparatus for performing handoff synchronization betweensynchronous and asynchronous base stations in a CDMA wirelesscommunication system.

[0004] II. Description of the Related Art

[0005]FIG. 1a is an exemplifying embodiment of a wireless communicationsystem having a terrestrial wireless network 140 in communication with awireless mobile station 110. Wireless network 140 is shown with twowireless base stations 120 and 130 such that mobile station 110 cancommunicate with either and can handoff between the two. The wirelesscommunication channel through which information signals travel frommobile station 110 to a base station 120 or 130 is known as a reverselink. The wireless communication channel through which informationsignals travel from a base station 120 or 130 to mobile station 110 isknown as a forward link.

[0006] Though only one serving base station is shown, an MS maycommunicate with multiple serving base stations in soft handoff and mayestablish a handoff to multiple target base stations. Mobile station 110communicates exclusively with one or more serving base stations 120before handoff is established. After handoff is established, mobilestation 110 may communicate in “soft handoff” with both serving basestations 120 and target base stations 130. Alternatively, following a“hard handoff,” mobile station 110 communicates exclusively with targetbase stations 130. In reality, typical wireless communication systemsmay have many more mobile stations and base stations than shown.Wireless network 140 includes one or more base station controllers orBSC's (not shown) and one or more mobile switching centers or MSC's (notshown). Serving base station 120 may be connected to a different BSC andMSC than target base station 130, or the two base stations may share thesame BSC and MSC.

[0007] Mobile station 110 may be any of a number of different types ofwireless communication devices such as a portable phone, a wirelesscommunication module incorporated into a portable computer, or a fixedlocation communication module such as might be found in a wireless localloop or meter reading system. In the most general embodiment, mobilestations may be any type of communication unit. For example, the mobilestations can be hand-held personal communication system units, portabledata units such as personal data assistants, or fixed location dataunits such as meter reading equipment.

[0008] In an exemplary embodiment, mobile station 110 communicates withserving base station 120 and target base station 130 using code divisionmultiple access (CDMA) techniques. An industry standard for a wirelesssystem using code division multiple access (CDMA) is set forth in theTIA/EIA Interim Standard entitled “Mobile station—Base StationCompatibility Standard for Dual-Mode Wideband Spread Spectrum CellularSystem”, TIA/EIA/IS-95, and its progeny (collectively referred to hereinas IS-95), the contents of which are also incorporated herein byreference. More information concerning a code division multiple accesscommunication system is disclosed in U.S. Pat. No. 4,901,307, entitled“SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE ORTERRESTRIAL REPEATERS”, assigned to the assignee of the presentinvention and incorporated in its entirety herein by this reference.

[0009] Third-generation CDMA wireless communications systems have alsobeen proposed. The cdma2000 ITU-R Radio Transmission Technology (RTT)Candidate Submission proposal forwarded by the TelecommunicationsIndustry Association (TIA) to the International Telecommunication Union(ITU) for consideration for the IMT-2000 CDMA standard is an example ofsuch a third-generation wireless communication system. The standard forcdma2000 is given in draft versions of IS-2000 and has been approved bythe TIA. The cdma2000 proposal is compatible with IS-95 systems in manyways. For example, in both the cdma2000 and IS-95 systems, each basestation time-synchronizes its operation with other base stations in thesystem. Typically, the base stations synchronize operation to auniversal time reference such as Global Positioning Satellites (GPS)signaling; however, other mechanisms can be used. Based upon thesynchronizing time reference, each base station in a given geographicalarea is assigned a sequence offset of a common pseudo noise (PN) pilotsequence. For example, according to IS95, a PN sequence having 215 chipsand repeating every 26.67 milliseconds (ms) is transmitted as a pilotsignal by each base station. The pilot PN sequence is transmitted byeach base station at one of 512 possible PN sequence offsets. Each basestation transmits the pilot signal continually, which enables mobilestations to identify the base station's transmissions as well as forother functions.

[0010] In addition to the pilot PN sequence, each base station in anIS-95 system transmits a “sync channel” signal that is synchronized tothe pilot PN signal. The sync channel contains information such as thePN offset of the base station and the CDMA System Time used by the basestation. When an IS95 mobile station first powers on, it must obtain theCDMA System Time from an IS-95 base station before it can communicatewith any synchronous base station. To obtain CDMA System Time, an IS-95mobile station performs a two-step acquisition procedure. First, themobile station performs a full search for the pilot PN sequencetransmitted by any synchronous base station. This search may beperformed over the entire 26.67 millisecond pilot PN code space. Uponlocating a pilot PN signal, the mobile station then decodes a syncchannel message from that base station's sync channel. Once a mobilestation successfully reads the sync channel, the mobile station has areference CDMA System Time that is accurate to within the time it takesfor the base station's signal to reach the mobile station. Because thistime is dependent on the unknown length of the signal propagation pathbetween the mobile station and the base station, it is called the pathdelay uncertainty. Because the path delay uncertainty is very smallcompared to the 512 possible pilot PN offsets used in IS95, the mobilestation can unambiguously distinguish signals transmitted by differentbase stations by their PN offsets.

[0011] Base station time-synchronization as provided in the cdma2000 andIS-95 systems has many advantages with respect to system acquisition andhandoff completion time. Synchronized base stations and time-shiftedcommon pilot signals as discussed above permit a fast one-stepcorrelation for system acquisition and detection of neighboring basestations. Once the mobile station has acquired one base station, it candetermine system time that is the same for all neighboring synchronousbase stations. In this case, there is no need to adjust the timing ofeach individual mobile station during a handoff between synchronous basestations. Additionally, the mobile station does not need to decode anysignal from the new base station in order to obtain rough timinginformation prior to handing off.

[0012] Another recently-proposed 3G communication system is referred toas W-CDMA. One example of a W-CDMA system is described in the ETSITerrestrial Radio Access (UTRA) International Telecommunications Union(ITU) Radio Transmission Technology (RTT) Candidate Submission forwardedby ETSI to the ITU for consideration for the IMT-2000 CDMA standard. Thebase stations in a W-CDMA system operate asynchronously. That is, theW-CDMA base stations do not all share a common universal time reference.Different base stations are not time-aligned. Consequently, a W-CDMAbase station may not be identified by its pilot signal offset alone.Also, once the system time of one base station is determined, thiscannot be used to estimate the system time of a neighboring basestation. For this reason, mobiles in a W-CDMA system use a three-stepPERCH acquisition procedure to synchronize with each base station in thesystem. Each step in the acquisition procedure identifies a differentcode within a frame structure called a PERCH channel.

[0013]FIG. 1b illustrates the parts of a frame transmitted on the W-CDMAPERCH channel. The PERCH channel is transmitted by each base station ina W-CDMA communication system and permits mobile stations to acquiresynchronization with each base station. A frame is 10 milliseconds induration and consists of 40,960 chips. A frame is divided into 16 slots,each slot having 2560 chips. Each slot can then be thought of as beingdivided into 10 consecutive parts, each part consisting of 256 chips.For the purposes of this disclosure, the 10 parts of each slot arenumbered from 1 to 10, with 1 being the earliest transmitted 256 chipsof each slot.

[0014] The first 256 chips (part 1) of each slot in the frame consist oftwo synchronization codes transmitted simultaneously. The first of thetwo codes is the primary synchronization code (PSC) sequence. The PSCsequence is the same sequence of 256 chips for every slot and for everybase station in a W-CDMA system. The second of the codes transmitted inpart 1 is the secondary synchronization code (SSC). The SSC sequencesidentify the timing of the parts in each slot, as well as the code groupto which the transmitting base station belongs.

[0015] Parts 2 through 6 of each slot include broadcast data such as thesystem identity of the transmitting base station and other informationthat is of general use to all mobile stations in communication with thatbase station. Parts 7 through 10 of each slot are used to carry a pilotcode signal that is generated in accordance with an Orthogonal Gold codeas defined by the aforementioned UTRA specification.

[0016] Mobile stations in a W-CDMA system employ a 3-step acquisitionprocedure, in which each step requires multiple parallel correlations toidentify the timing of different synchronization codes. The mobilestation first synchronizes to the PSC to identify the boundaries betweenthe different parts of each slot. Second, once the boundaries betweenparts have been identified, the mobile station uses the timinginformation derived from the primary synchronization channel to deriveslot timing information and a Group ID from the secondarysynchronization channel. Third, the mobile station decodes the pilotcode signal in order to determine the identity of the transmitting basestation and to identify the boundaries between frames. Once this processis complete, the mobile station may also be required to decodeinformation from the broadcast channel (BCCH) to unambiguously determinethe system time of the transmitting base station. The BCCH transmitstiming information that can be used to identify the W-CDMA frame number.Because the frame boundaries are aligned with W-CDMA System Time, theW-CDMA frame number combined with the frame timing information can beused to determine the W-CDMA System Time of a W-CDMA base station.

[0017] Recently, a combined CDMA IMT-2000 standard has been proposed inwhich cdma2000-compliant equipment and W-CDMA-compliant equipment may beoptionally supported by any manufacturer. Thus, it is expected thatsynchronous base stations of a cdma2000-compliant system will begeographically located near asynchronous base stations of aW-CDMA-compliant system. This creates a need to be able to handoff amobile station that supports both cdma2000 and W-CDMA operation betweenthe asynchronous base stations of a W-CDMA system and the synchronousbase stations of a cdma2000 system, and vice versa.

[0018] U.S. Pat. No. 5,267,261 entitled “MOBILE STATION ASSISTED SOFTHANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM,” which is assigned tothe assignee of the present invention and which is incorporated herein,discloses a method and system for providing communication with themobile station through more than one base station during the handoffprocess. Further information concerning handoff is disclosed in U.S.Pat. No. 5,101,501, entitled “METHOD AND SYSTEM FOR PROVIDING A SOFTHANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM”, U.S.Pat. No. 5,640,414, entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN ACDMA CELLULAR COMMUNICATIONS SYSTEM”, and U.S. Pat. No. 5,625,876entitled “METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OFA COMMON BASE STATION,” each of which is assigned to the assignee of thepresent invention and incorporated in its entirety herein by thisreference. The subject matter of U.S. Pat. No. 5,625,876 concernsso-called “softer handoff.” For the purposes of this document, the term“soft handoff” is intended to include both “soft handoff” and “softerhandoff.”

[0019] Each base station is associated with a set of neighboring basestations surrounding the base station. Due to the physical proximity ofthe coverage areas of the neighboring base stations to the coverage areaof the active base station, the mobile stations which are communicatingwith the active base station are more likely to handoff to one of theneighboring base stations than to other base stations in the system. Inthe IS-95 and cdma2000 systems, the base station identifies theneighboring base stations to the mobile stations with which it hasestablished communication using a neighbor list message. The neighborlist message identifies a neighboring base station according to the PNsequence offset at which it transmits the pilot signal. In the IS-95 andcdma2000 systems, there is a one-to-one correspondence in a givengeographical area between a base station and a PN sequence offset. Inother words, two base stations operating in the same geographical areado not both use the same PN sequence offset. Thus, a base station in theIS-95 or cdma2000 system can be uniquely identified in a geographicalregion by its PN sequence offset.

[0020] The mobile station uses the neighbor list to limit the space overwhich it searches for handoff candidates. Because the searching processis so resource intensive, it is advantageous to avoid performing asearch over the entire set of possible PN sequence offsets. By using theneighbor list, the mobile station can concentrate its resources on thosePN sequence offsets which are most likely to correspond to useful signalpaths.

[0021] A typical IS-95 or cdma2000 neighbor acquisition operation ispractical so long as each base station's timing remains synchronous withrespect to the others. However, in some systems such as W-CDMA,advantages are achieved by decoupling operation of the system from asynchronizing reference. For example, in a system which is deployedunderground, such as in a subway system, it can be difficult to receivea universal time synchronization signal using GPS. Even where strong GPSsignals are available, it is perceived as desirable in some politicalclimates to decouple system operation from the U.S. Government GPSsystem. There may be other reasons for decoupling operation of thesystem from a synchronizing reference.

[0022] In a system where one or more of the base stations operateasynchronously with respect to other base stations in the system, thebase stations cannot be readily distinguished from one another basedmerely upon a relative time offset (typically measured as a relative PNsequence offset) because a relative time offset between the basestations cannot be established without the use of a universal timereference. Thus, when a mobile station is in communication with anasynchronous base station, and has not been recently in communicationwith a synchronous base station, the mobile station is unlikely to havesystem time information of the synchronous base stations to a sufficientaccuracy.

[0023] For example, suppose a mobile station has been in the coveragearea of an asynchronous base station and is moving into the coveragearea of a synchronous base station. Further suppose that the mobilestation is able to detect the pilot signals of two different synchronousbase stations by determining their relative PN sequence offsets. Unlessthe mobile station already knows system time of the synchronous basestations to a sufficient accuracy, the mobile station may not be able todistinguish the pilot signal of one base station from the pilot signalof another. For example, even if the mobile station can determine thereceived pilot signals belong to two synchronous base stations, themobile station may be unable to identify either based on their pilotsignals alone.

[0024] In a conventional IS-95 or cdma2000 system, once the forwardpilot channel is acquired, the mobile station can then demodulate theforward synchronization channel. This is possible because the forwardsync channel timing is such that its frame boundary is always aligned tothe beginning of the PN sequence of the forward pilot channel. In otherwords, the forward sync channel frame boundary is always offset fromsystem time by the same number of PN chips as the PN sequence offset ofthe corresponding forward pilot channel. The forward sync channelcarries a sync channel message which includes overhead information suchas system identification, system time, the base station's PN sequenceoffset, and several other items of useful information. Afterdemodulating the sync channel message, the mobile station adjusts itsinternal timing according to the PN offset and system time sent in thesync channel message as described in IS-95.

[0025] The conventional sync channel is transmitted at a low data rate(for example, 1200 bps in IS95), and the sync channel message contains alarge amount of overhead information that must be demodulated on aframe-by-frame basis. Consequently, determining the system identity ofthe transmitting base station via the sync channel message may take aslong as 800 milliseconds. This delay can undesirably affect the timingof a handoff from the asynchronous base station to the synchronous basestation, particularly in a fading environment. In some instances, thedelay associated with the mobile station having to determine the systemidentification of the synchronous target base station(s) by demodulatinga conventional sync channel message would be unacceptably long, causingdegradation or even dropping of a call in progress.

[0026] In a cdma2000 system, a mobile station communicating with aserving base station can quickly and efficiently search for signals ofneighboring base stations. Once a mobile station receives a neighborlist message from the serving base station, the mobile station candetermine a time offset search window within which the signaltransmitted from a target base station will arrive at the mobilestation. Because the width of the search window is roughly proportionalto the average time needed to locate a signal within the window, anarrow search window is preferable to a wide search window. In acdma2000 system, the narrowness of the search window is limited only bythe relatively small path delay uncertainty between the mobile stationand the two different base stations. Additionally, once the mobilestation in a cdma2000 system finds the signal of the target base stationwithin the search range, the mobile station can uniquely identify thetarget base station by its PN offset.

[0027] In contrast, in a W-CDMA system in which the base stations maynot be synchronized with each other, a mobile station cannot use timinginformation from a serving base station to accurately predict thearrival time of a signal from a target base station. In order to locatethe signal of an unsynchronized target base station, a W-CDMA mobilestation must perform the full three-step PERCH acquisition proceduredescribed above. This acquisition procedure takes substantially longerthan searching within a narrow window for a cdma2000 signal, andtherefore increases the time required to establish a handoff in a W-CDMAsystem.

[0028] In the worst case, the timing of different W-CDMA base stationsmay vary so widely that the mobile station will not know the transmitframe number of the target base station even after performing thethree-step PERCH acquisition procedure. This ambiguity can occur if thebase stations within the W-CDMA wireless system might be as much as ahalf-frame period out of synchronization. In such a system, the mobilestation must monitor information transmitted on the target basestation's broadcast channel to identify the frame number of the targetbase station. This extra decoding step further increases the timerequired to establish a handoff in a W-CDMA system.

[0029] The lack of synchronization in asynchronous systems such asW-CDMA systems presents difficulties in establishing handoff betweensynchronous and asynchronous base stations. Such problems arise when amobile station is located in a region that lies within the coverage areaof both synchronous and asynchronous wireless systems. Such a region canexist along the border between a synchronous and an asynchronous system,or may exist throughout a coverage area serviced by overlappingsynchronous and asynchronous wireless systems, such as W-CDMA andcdma2000 systems.

[0030] In order to perform handoff from an asynchronous system to asynchronous system, a mobile station must hand off from an asynchronousbase station to a synchronous base station. Because the asynchronousserving base station, unlike the synchronous target base station, is notsynchronized to a universal time reference, the asynchronous basestation cannot provide timing information for the target base station tothe mobile station. In the absence of timing information for thesynchronous target base station, the mobile station must perform a fullpilot search and then read the sync channel in order determine thesystem timing of the target base station. Performing a full pilot searchand reading the sync channel of the target base station is undesirable,because it increases the time required to establish a handoff.

[0031] In order to perform handoff from a synchronous system to anasynchronous system, a mobile station must hand off from a synchronousbase station to an asynchronous base station. Because the asynchronoustarget base station, unlike the synchronous serving base station, is notsynchronized to a universal time reference, the synchronous base stationcannot provide timing information for the target base station to themobile station. In the absence of timing information for theasynchronous target base station, the mobile station must perform thefull three-step PERCH acquisition procedure to determine frame timing ofthe asynchronous target base station. In order to find the full systemtime of the asynchronous base station, the mobile station must then readthe broadcast channel information transmitted by the asynchronous targetbase station. Performing the three-step PERCH acquisition procedure andreading the broadcast channel of the target base station is undesirable,because it increases the time required to establish a handoff.

[0032] Thus, there is a need for an improved method and system forfacilitating handoff between asynchronous and synchronous base stationsthat avoids the undesirable delays associated with determining thesystem time of the target base station.

SUMMARY OF THE INVENTION

[0033] An aspect of the present invention enables faster determinationof system timing of a target base station in a mixed network ofsynchronous and asynchronous base stations. By utilizing informationabout the relative timing between synchronous and asynchronous basestations, a mobile station determines the system time of the target basestation in an advantageously short time. Handoff of a mobile stationfrom an asynchronous serving base station to a synchronous target basestation is accomplished without performing a full pilot search for thetarget base station and without decoding a full sync channel packet.Handoff of a mobile station from a synchronous serving base station toan asynchronous target base station is accomplished without performing afull three-step PERCH acquisition procedure for the target base stationand often without decoding information from the broadcast channel.

[0034] An aspect of the present invention utilizes knowledge thatasynchronous base stations within a single wireless network may besynchronized to each other. Also, the oscillators in the asynchronousnetwork are expected to be very stable in relation to the universal timereference used in synchronous wireless networks. In other words, anydifference between the system time of an asynchronous base station andthe system time of a synchronous base station remains nearly constantover short periods.

[0035] The timing of asynchronous base stations relative to each otherand relative to neighboring synchronous base stations may be measuredevery time a mobile station establishes a handoff between them. As longas handoffs between asynchronous and synchronous base stations occurfrequently compared with the drift characteristics of oscillators withinthe asynchronous base stations, an accurate estimate of the relativesystem timing between the base stations can be maintained.

[0036] If handoff between base stations in a completely asynchronousnetwork occurs often enough, the asynchronous base stations may also bekept nearly synchronized with each other. In a similar fashion, frequenthandoffs between the base stations of a synchronous network and anasynchronous network may enable all of the asynchronous base stations tomaintain system timing that is very near the timing provided by theuniversal time reference used by the synchronous network. U.S. Pat. No.5,872,774, entitled “MOBILE STATION ASSISTED TIMING SYNCHRONIZATION IN ACDMA COMMUNICATION SYSTEM,” hereinafter referred to as the '774 patent,assigned to the assignee of the present invention and incorporated byreference herein, discloses an exemplary method of utilizing handoffs tosynchronize base stations to each other without the use of a universaltime reference. The techniques disclosed in the '774 patent may be usedto synchronize synchronous base stations to each other or to synchronizeasynchronous base stations to each other. In addition, the techniquesdisclosed in the '774 patent may be used to synchronize the basestations of an asynchronous network to the base stations of aneighboring synchronous network.

[0037] Even without using the techniques disclosed in the '774 patent,timing information gathered by base stations communicating with a commonmobile station may be used to very accurately estimate the relativesystem time used by each of those base stations. If the difference insystem time between an asynchronous base station and a synchronous basestation is very small or is known to within a great degree of accuracy,a mobile station may use the system time of the serving base station toaccurately estimate the phase of the forward link signal received fromthe target base station. This is true whether establishing handoff froma synchronous base station to an asynchronous base station or viceversa.

[0038] The accurate estimation of the timing of the target basestation's forward link signal enables the mobile station to quickly andefficiently search for the forward link signal of the target basestation and to then unambiguously determine the system time of thetarget base station. If the target base station is synchronous themobile station acquires the target base station's forward link signaland system timing without performing a full pilot search and withoutreading the sync channel message. If the target base station isasynchronous, then the mobile station acquires the target base station'sforward link signal and system timing without performing a full pilotsearch and without reading the sync channel message.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The features, objects, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

[0040]FIG. 1a is an exemplifying embodiment of a wireless communicationsystem.

[0041]FIG. 1b illustrates the parts of a frame transmitted on a W-CDMAPERCH channel.

[0042]FIG. 2 is a diagram showing the timing associated with performinghandoff of a mobile station from a synchronous serving base station to asynchronous target base station.

[0043]FIG. 3 is a flowchart showing the steps of an exemplary method ofestablishing handoff of a mobile station from a synchronous serving basestation to a synchronous target base station.

[0044]FIG. 4 is a diagram showing the timing associated with performinghandoff of a mobile station from a serving base station to anasynchronous target base station.

[0045]FIG. 5 is a flowchart showing the steps of an exemplary method ofestablishing handoff of a mobile station from a serving base station toan asynchronous target base station.

[0046]FIG. 6 is a diagram showing the timing associated with performinghandoff of a mobile station from an asynchronous serving base station toa synchronous target base station.

[0047]FIG. 7 is a flowchart showing the steps of an exemplary method ofestablishing handoff of a mobile station from an asynchronous servingbase station to a synchronous target base station.

[0048]FIG. 8 is a simplified timing diagram of a short sync channelstructure transmitted by synchronous base stations in accordance with anembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049]FIG. 2 is a diagram showing the timing associated with performinghandoff of a mobile station (MS) between a first synchronous basestation (BTS1) and a second synchronous base station (BTS2). In anexemplary embodiment, the handoff may be a “hard” handoff, in which theMS does not receive information from BTS1 once a communication link hasbeen established with BTS2. More advantageously, the handoff is a “soft”handoff, in which the MS receives signals from both BTS1 and BTS2simultaneously once a handoff is established.

[0050] Though only one serving base station is shown, an MS maycommunicate with multiple serving base stations in soft handoff and mayestablish a handoff to multiple target base stations. In an exemplaryembodiment, both BTS1 and BTS2 are synchronized to an external timereference, for example GPS. Each of BTS1 and BTS2 uses the external timereference to establish a CDMA System Time. BTS1 establishes a BTS1 CDMASystem Time, and BTS2 establishes a BTS2 CDMA System Time. Because bothBTS's use the same time reference 202, the BTS1 CDMA System Time will bethe same as BTS2 CDMA System Time. The CDMA System Time used by thesynchronous base stations is referred to herein as Synchronous CDMASystem Time.

[0051] During acquisition of the forward link signal of a serving basestation (BTS1), the mobile station establishes an MS CDMA System Timebased on timing information within the BTS1 forward link signal. Due tothe forward link propagation delay of signals transmitted by BTS1 to theMS, the MS CDMA System Time used by the MS lags behind the SynchronousCDMA System Time. The lag time between the MS CDMA System Time andSynchronous CDMA System Time is equal to the one-way propagation delayin the signal (T1) 204.

[0052] The PN phase of reverse link signals transmitted by the MS arealigned with MS CDMA System Time. BTS1 measures the PN phase of thereverse link signals 212 received from the MS and uses the reverse linkPN phase to determine the round-trip delay time (RTD1) 206 between BTS1and the MS. Once RTD1 206 is known, then T1 204 is known to be half ofRTD1 206.

[0053] In an exemplary embodiment, the MS may receive forward linksignals from multiple base stations. For example, if the MS is in ahandoff region between BTS1 and BTS2, the MS will receive forward linksignals from both base stations. In an exemplary embodiment, the servingbase station (BTS1) sends a Neighbor List of nearby “neighbor basestations” to the MS. The Neighbor List includes identifyingcharacteristics of the forward link signal of each neighbor basestation. In an exemplary embodiment, the Neighbor List contains a pilotsignal offset for each base station in the Neighbor List that the MS canuse to distinguish forward link signals emanating from any base stationin the Neighbor List.

[0054] With the information in the Neighbor List, the MS searches forand identifies the forward link signals of other base stations such asBTS2. The MS also measures the relative timing of the received forwardlink signal from BTS2 as compared to the forward link signal from BTS1.Differences in relative timing are due to differences in signal pathlength between the MS and each base station. In an exemplary embodiment,the MS measures the relative phase of pilot signals transmitted by BTS1and BTS2 to determine the relative timing of the signals received fromboth base stations. The MS measures the difference in CDMA System Timeindicated by the phase difference between the pilot signals transmittedby BTS1 and BTS2. In an exemplary embodiment, the MS sends a PilotStrength Measurement Message (PSMM) to the serving base station (BTS1)indicating the phase difference between the BTS1 and BTS2 pilot signalsreceived at the MS. The phase difference is used by the wireless networkto determine the time difference (ΔT) 208 between arrival at the MS offorward link signals transmitted simultaneously from BTS1 and BTS2.Using this information, the wireless network can then determine theone-way propagation delay from BTS2 to the MS (T2) 210. In an exemplaryembodiment, T2 is determined using the equation:

T2=T1+ΔT

[0055] where T1 is the propagation delay from the MS to BTS1 204, T2 isthe one-way propagation delay from the MS to BTS2 210, and ΔT is thetime difference between arrival at the MS of forward link signalstransmitted simultaneously from BTS1 and BTS2 210.

[0056] In an exemplary embodiment, BTS1 and BTS2 can adjust the phase oftheir transmissions of forward link frames in 1.25 millisecond stepscalled frame offsets. All forward link traffic channel frames from BTS2to the MS are transmitted using the same frame offset as used by BTS1.

[0057] In an exemplary embodiment, the MS transmits reverse link framesat specific time intervals according to MS CDMA System Time. Thewireless network knows these intervals, and uses that timing informationand reverse link frame timing from BTS1 to predict the arrival time 214of reverse link frames at the receiver in BTS2. For example, if a frametransmitted by the MS arrives at BTS1 at time T, then the same framewill arrive at BTS2 at time T+ΔT. Once reverse link frame timing hasbeen determined, BTS2 can easily and efficiently search for the reverselink signal transmitted by the MS.

[0058]FIG. 3 is a flowchart showing the steps of an exemplary method ofestablishing handoff of a mobile station from a synchronous serving basestation (BTS2) to a synchronous target base station (BTS1). In anexemplary embodiment, both synchronous base stations (BTS1 and BTS2) aresynchronized to a common universal time reference. Some time afterestablishing a link between BTS1 and the MS, BTS1 sends a Neighbor Listto the MS at step 300. As described above, the Neighbor List includesidentifying characteristics of the forward link signal of each neighborbase station. At step 302, BTS1 measures RTD1 (206 in FIG. 2) todetermine T1 (204 in FIG. 2). At step 304, the MS searches for a pilotsignal that has the PN offset associated with BTS2. The width of thesearch window used by the MS is derived from the path delay uncertainty.At step 306, the MS acquires the pilot signal from BTS2 and measures therelative signal phase of the forward link signal received from BTS2 ascompared with the signal phase of the forward link signal received fromBTS1. The relative PN signal phases may be used by the synchronousnetwork to determine ΔT (208 in FIG. 2). At step 308, the MS sends thecomparative signal phase information measured at step 306 to BTS1 in aPilot Strength Measurement Message (PSMM).

[0059] In an exemplary embodiment, BTS2 then begins transmitting forwardlink frames to the MS using the same frame offset used by BTS1. At step310, BTS2 sets its search window based on the estimated arrival time ofreverse link signals transmitted by the MS. The wireless network cansend a handoff direction message to the MS either before BTS2 beginstransmitting or after BTS2 begins transmitting. If the handoff directionmessage is sent after BTS2 begins transmitting, then the handoffdirection message is preferably transmitted from both BTS1 and BTS2. Theestimated arrival time is determined by adding ΔT (208 in FIG. 2) to themeasured arrival time of MS transmissions at BTS1. At step 312, BTS2acquires the reverse link signal transmitted by the MS. Then, at step314, BTS2 adjusts (deskews) its reverse link demodulation timingaccording to any differences between the estimated arrival timementioned above and the actual arrival time of reverse link signals fromthe MS.

[0060]FIG. 4 is a diagram showing the timing associated with performinghandoff of a mobile station (MS) from a serving base station (BTS1) toan asynchronous target base station (BTS2). The handoff may be a hardhandoff or a soft handoff. Though only one serving base station isshown, an MS may communicate with multiple serving and base stations insoft handoff and may perform handoff to multiple target base stations.

[0061] In FIG. 4, the CDMA System Time used by each base station BTS1and BTS2 is shown as offset from a universal time reference 401. TheCDMA System Time used by BTS1 is called BTS1 CDMA System Time, and isoffset from universal time reference 401 by a timing error TE1 422. TheCDMA System Time used by BTS2 is called BTS2 CDMA System Time, and isoffset from universal time reference 401 by a timing error TE2 424. Thedifference in CDMA System Times between the two base stations (ΔTE) 417is equal to TE1-TE2 and is called the relative timing error of BTS2 inrelation to BTS1.

[0062] In an exemplary embodiment, BTS2 is asynchronous and BTS1 may besynchronous or asynchronous. If BTS1 is synchronous, then TE1 will equalzero. If BTS1 is asynchronous, then TE1 will generally not equal zero.In either case TE2 424 and ΔTE 417 will generally not be zero.

[0063] In an exemplary embodiment, timing information gathered duringhandoffs is used to maintain estimates of TE1 422, TE2 424, and ΔTE 417.Because of drift in the oscillators in an asynchronous base station, thetiming error estimates can be expected to become steadily less accurateafter they are measured during handoffs. In other words, as more timepasses after ΔTE 417 is measured in a handoff, the estimate of ΔTE 417can be expected to grow less reliable. In an exemplary embodiment, eachbase station maintains timing error estimates for all its neighbor basestations and transmits the timing errors and expected timing erroraccuracies in periodically broadcast Neighbor Lists.

[0064] During acquisition of the forward link signal of a serving basestation (BTS1), the mobile station establishes an MS CDMA System Timebased on timing information within the BTS1 forward link signal. Becauseof the propagation delay in the forward link channel from BTS1 to theMS, the MS CDMA System Time used by the MS will lag behind BTS1 CDMASystem Time. The lag time between the MS CDMA System Time and BTS1 CDMASystem Time is equal to the one-way propagation delay in the signal (T1)404.

[0065] The PN phase of reverse link signals transmitted by the MS arealigned with MS CDMA System Time. BTS1 measures the PN phase of thereverse link signals 412 received from the MS and uses the reverse linkPN phase to determine the round-trip delay time (RTD1) 406 between BTS1and the MS. Once RTD1 406 is known, then T1 404 is known to be half ofRTD1 406.

[0066] In an exemplary embodiment, the serving base station (BTS1) sendsa Neighbor List of nearby “neighbor base stations” to the MS. TheNeighbor List includes identifying characteristics of the forward linksignal of each neighbor base station. For an asynchronous neighbor basestation such as BTS2, the Neighbor List includes such information aspilot code, an estimate of ΔTE 417, and the expected accuracy of the ΔTEestimate. The Neighbor List may also include path delay uncertaintyinformation.

[0067] In an alternate embodiment, each asynchronous base stationperiodically adjusts its CDMA System Time after performing a handoffwith a “master” base station as described in the aforementioned '774patent. After performing each such adjustment, the asynchronous basestation adds an offset to each timing error estimate in its broadcastNeighbor List.

[0068] If the estimate of ΔTE 417 for BTS2 in the Neighbor List is knownto be accurate, such as when it has been updated recently, the MS doesnot perform the full three-step PERCH acquisition procedure. Forexample, if the inaccuracy of the ΔTE estimate is less than half of a10-millisecond frame, then the MS can search for the pilot code of BTS2within a search window based on the inaccuracy of the ΔTE estimate.Alternatively, the MS can search for the primary synchronization code(PSC) or the secondary synchronization code (SSC) in order to determineframe timing. The MS may additionally use the path delay uncertainty ofthe system to determine the width of the search window used to searchfor the PSC, SSC, or pilot code of BTS2.

[0069] If no estimate of ΔTE 417 is available for BTS2, or if it is notknown to within a half of a frame, then the MS must perform a fullthree-step PERCH acquisition procedure to identify the signaltransmitted by BTS2. If, after performing the three-step PERCHacquisition procedure, BTS2 CDMA System Time is ambiguous, then the MSmust read the broadcast channel of BTS2 to determine BTS2 CDMA SystemTime.

[0070] In an exemplary embodiment, the MS then sends a Pilot StrengthMeasurement Message (PSMM) to the serving base station (BTS1). The PSMMindicates the strength of the signal received from BTS2 and thedifference 408 between BTS1 CDMA System Time and BTS2 CDMA System Timeas measured at the MS.

[0071] Because the (ΔTE) 417 is not known, the one-way propagation delayfrom BTS2 to the MS (T2) 410 can only be estimated from T1 404 and ΔT408. However, an offset TO2 420 can be accurately determined that willenable BTS2 to transmit forward link frames such that they arrive at theMS at approximately the same time as frames transmitted from BTS1. In anexemplary embodiment, the BTS2 forward link frame offset TO2 420 isdetermined using the equation:

TO2=TO1−ΔT

[0072] where TO1 418 is the forward link frame offset used by BTS1, TO2420 is the forward link frame offset used BTS2, and ΔT is the timeoffset 408 between BTS1 CDMA System Time and BTS2 CDMA System Timemeasured at the MS.

[0073] In an exemplary embodiment, BTS2 is an asynchronous base stationand can adjust its forward link frame offset TO2 420 to relatively fineresolution compared to the 1.25 millisecond increments used by IS-95synchronous base stations. For example, asynchronous base station BTS2may be able to adjust its forward link frame offset TO2 420 inincrements of one Walsh symbol of 64 chips (15.6 microseconds at 4.096megachips-per-second). BTS2 transmits forward link frames to the MSusing offset TO2 420.

[0074] In an exemplary embodiment, the MS begins demodulating theforward link signal from BTS2 after receiving a handoff directionmessage from BTS1. BTS2 may begin transmitting forward link frames tothe MS either before or after the handoff direction message istransmitted without departing from the described method.

[0075] Because the T2 410 is not accurately known, the time that reverselink frames transmitted by the MS will arrive at BTS2 can only beapproximated. In order to acquire the reverse link signal from the MS,BTS2 searches for the reverse link signal within an estimated searchwindow. The width of the estimated search window is based on the pathdelay uncertainty. The location of the estimated search window is basedon the measured RTD1 406.

[0076] As described above, the MS transmits its reverse link signalsusing a PN phase that is aligned with MS CDMA System Time. Once BTS2acquires the reverse link signal from the MS, BTS2 measures differenceTm 426 between BTS2 CDMA System Time 416 and the MS CDMA System Time ofthe reverse link signal as measured at the receiver of BTS2 414. Thewireless network can then determine the difference ΔT 417 between BTS1CDMA System Time 402 and BTS2 CDMA System Time 416. ΔT 417 may bedetermined according to the equation:${\Delta \quad {TE}} = {{T1} + \frac{{\Delta \quad T} - {Tm}}{2}}$

[0077] where ΔTE is the timing error between BTS1 CDMA System Time 402and BTS2 CDMA System Time 416, T1 404 is the one-way propagation delayfrom BTS1 to the MS, ΔT is the time offset 408 between BTS1 CDMA SystemTime and BTS2 CDMA System Time measured at the MS, and Tm is thedifference 426 between BTS2 CDMA System Time 416 and the MS CDMA SystemTime of the reverse link signal as measured at the receiver of BTS2. Thewireless network uses the new ΔTE to update the Neighbor List messagessubsequently transmitted by BTS1 and BTS2.

[0078]FIG. 5 is a flowchart showing the steps of an exemplary method ofestablishing handoff of a mobile station from a serving base station(BTS1) to an asynchronous target base station (BTS2). At step 500, BTS1sends a Neighbor List to the MS. In an exemplary embodiment, theNeighbor List is sent by BTS1 in one or more broadcast messages. TheNeighbor List includes forward link characteristics for each neighborbase station such as pilot code, estimated timing error, and expectedaccuracy of the estimated timing error.

[0079] At step 502, BTS1 measures RTD1 (406 in FIG. 4) to determine T1(404 in FIG. 4). At step 504, the MS evaluates the expected accuracy ofthe estimated timing error associated with BTS2. If the estimated timingerror is known to be accurate, then at step 512 the MS performs anabbreviated search for the forward link signal of BTS2. In an exemplaryembodiment, the MS searches for the pilot code of BTS2 within a searchwindow based on the inaccuracy of the ΔTE estimate and the path delayuncertainty of the system. In an alternate embodiment, the MS searchesfor primary synchronization code of BTS2 within the search window. Inanother alternate embodiment, the MS searches for secondarysynchronization code of BTS2 within the search window.

[0080] If the estimated timing error for BTS2 is not accurately known,then at step 506, the MS performs a three-step PERCH acquisitionprocedure to acquire the forward link signal of BTS2. At step 508, theMS evaluates whether BTS2 CDMA System Time can be unambiguouslydetermined from the PERCH channel timing. If not, then at step 510 theMS reads the BCCH signal to determine BTS2 CDMA System Time. Theevaluation at step 508 may be performed prior to step 506, in which casethe three-step PERCH acquisition procedure and decoding the BCCH signalmay be done simultaneously.

[0081] After performing step 510 or 512, or if BTS2 CDMA System Time isunambiguously determined at step 508, the MS measures the timing errorof BTS2 in relation to BTS1 at step 514. At step 516, the MS sendsinformation indicating the BTS2 timing error within a Pilot StrengthMeasurement Message (PSMM) to serving base station BTS1. The wirelessnetwork uses this timing error information to update the timing errorsent to mobile stations in subsequent Neighbor List messages.

[0082] In an exemplary embodiment, at step 518, BTS2 begins transmittingforward link frames to the MS at an offset TO2 (420 in FIG. 4). TO2 iscalculated as described above. The wireless network can send a handoffdirection message to the MS either before BTS2 begins transmitting orafter BTS2 begins transmitting. If the handoff direction message is sentafter BTS2 begins transmitting, then the handoff direction message ispreferably transmitted from both BTS1 and BTS2. At step 520, BTS2searches for the reverse link signal of the MS over an estimated searchwindow. The width of the estimated search window is based on the pathdelay uncertainty. The location of the estimated search window is basedon the measured RTD1 (406 in FIG. 4).

[0083] At step 522, BTS2 acquires the reverse link signal transmitted bythe MS. Then, at step 524, BTS2 adjusts (deskews) its reverse linkdemodulation timing according to the actual arrival time of reverse linksignals from the MS.

[0084]FIG. 6 is a diagram showing the timing associated with performinghandoff of a mobile station (MS) from an asynchronous serving basestation (BTS1) to a synchronous target base station (BTS2). The handoffmay be a hard handoff or a soft handoff. Though only one serving basestation is shown, an MS may communicate with multiple serving and basestations in soft handoff and may perform handoff to multiple target basestations.

[0085] In FIG. 6, the CDMA System Time used by BTS1 is shown as offsetfrom a universal time reference 601. BTS2 is a synchronous base stationand is therefore synchronized to the universal time reference 601. BTS1CDMA System Time is offset from universal time reference 601 by a timingerror TE1 622. BTS2 CDMA System Time is equal to the universal timereference. The timing error of BTS2 in relation to BTS1 is equal to TE1.

[0086] In an exemplary embodiment, timing information gathered duringhandoffs is used to maintain an estimate of TE1 622. Because of drift inthe oscillators in asynchronous base station BTS1, estimates of thetiming error between BTS1 and BTS2 can be expected to grow less accuratein the period following a measurement during handoff. In other words, asmore time passes after TE1 622 is measured in a handoff, the estimate ofTE1 622 can be expected to grow less reliable.

[0087] After a call is established between a mobile station (MS) and theserving base station (BTS1), the mobile station establishes an MS CDMASystem Time based on timing information received from BTS1. Because ofthe propagation delay in the forward link channel from BTS1 to the MS,the MS CDMA System Time used by the MS will lag behind BTS1 CDMA SystemTime. The lag time between the MS CDMA System Time and BTS1 CDMA SystemTime is equal to the one-way propagation delay in the signal (T1) 604.BTS1 measures the round-trip delay time (RTD1) 606 between BTS1 and theMS as described above.

[0088] When the MS sends a time-stamped signal 612 to BTS1, BTS1measures the arrival time of the MS information according to BTS1 CDMASystem Time. Using this timing information, BTS1 determines theround-trip-delay time (RTD1) 606 between BTS1 and the MS. Once RTD1 606is known, then T1 604 is known to be half of RTD1 606.

[0089] In an exemplary embodiment, the serving base station (BTS1) sendsa Neighbor List to the MS. The Neighbor List includes identifyingcharacteristics of the forward link signal of each neighbor basestation. For a synchronous neighbor base station such as BTS2, theNeighbor List includes such information as an estimated pilot offset andthe expected accuracy of the estimated pilot offset. The Neighbor Listmay also include path delay uncertainty information.

[0090] If the pilot offset for BTS2 in the Neighbor List is known to beaccurate, the MS searches for the pilot signal from BTS2 using a searchwindow based on the expected pilot offset inaccuracy. For example, ifthe inaccuracy of the ΔTE estimate is less than half of the time offsetbetween the next-closest pilot offset of a synchronous neighbor basestation, then the MS can search for the pilot signal of BTS2 within asearch window based on the inaccuracy of the pilot offset estimate. TheMS may additionally use the path delay uncertainty of the system todetermine the search window width. In an exemplary embodiment, the PNoffsets of the synchronous neighbor base stations of an asynchronousbase station are spaced far apart from each other in order to allowunambiguous identification of synchronous base station pilots using widesearch windows.

[0091] If the inaccuracy of the pilot offset estimate for BTS2 is large,then the MS searches for the BTS2 pilot over the entire pilot codesearch range of 26.67 milliseconds. After acquiring one or more pilotsignals, the MS reads the sync channel associated with one of the pilotsignals to obtain an estimate of CDMA System Time that is synchronizedto a universal time reference. Once CDMA System Time is obtained fromany synchronous base station, the MS can unambiguously determine thepilot offsets of any other pilot signals received from other synchronousbase stations. The MS then measures the difference between CDMA SystemTime received from BTS1 and BTS2 (shown as ΔT 608). In an exemplaryembodiment, the MS then sends a Pilot Strength Measurement Message(PSMM) to the serving base station (BTS1). Information in the PSMMindicates the strength of the signal received by the MS from BTS2 and ΔT608.

[0092] Because the TE1 622 is only known to within the path delayuncertainty of the system, the one-way propagation delay from BTS2 tothe MS (T2) 610 can only be estimated from T1 604 and ΔT 608. However,an offset TO2 620 can be accurately determined that would enable BTS2 totransmit forward link frames such that they arrive at the MS atapproximately the same time as frames transmitted from BTS1.

[0093] In an exemplary embodiment, a synchronous target base stationBTS2 can only adjust the forward link frame offset TO2 620 in incrementsof 1.25 milliseconds. In order to establish a handoff from anasynchronous serving base station BTS1 to a synchronous target basestation BTS2, it is generally necessary to adjust the transmit timing ofboth the serving and target base stations so that frames from both basestations arrive at the MS at nearly the same time. In an exemplaryembodiment, BTS2 chooses its frame offset in order to minimize the sizeof the timing adjustment that BTS1 must make. In other words, since TO2620 can only be adjusted in steps of 1.25 milliseconds, a value of TO2620 is chosen such that minimal change to TO1 618 will be required tosynchronize forward link transmissions. TO2 620 is chosen to minimize|TO2-TO1-ΔT|, where TO1 618 is the forward link frame offset used byBTS1, TO2 620 is the forward link frame offset used by BTS2, and ΔT isthe time offset 608 between BTS1 CDMA System Time and BTS2 CDMA SystemTime measured at the MS. After TO2 620 is chosen for BTS2, then amodified TO1′ 624 is determined that will minimize |TO2-TO1-ΔT| towithin a Walsh symbol (15.6 microseconds at 4.096 megachips-per-second).BTS1 then adjusts its transmit frame offset from the old TO1 618 to themodified TO1′ 624. BTS2 transmits forward link frames to the MS atoffset TO2.

[0094] In an exemplary embodiment, the MS begins demodulating theforward link signal from BTS2 after receiving a handoff directionmessage from BTS1. The order in which TO1 and TO2 are adjusted and thetime that the handoff direction message is transmitted may be variedwithout departing from the described method.

[0095] Because the T2 610 is not accurately known, the time that reverselink frames transmitted by the MS will arrive at BTS2 can only beapproximated. In order to acquire the reverse link signal from the MS,BTS2 searches for the reverse link signal of the MS within an estimatedsearch window. The width of the estimated search window is based on thepath delay uncertainty. The location of the estimated search window isbased on the measured RTD1 606.

[0096] As described above, the MS transmits its reverse link signalsusing a PN phase that is aligned with MS CDMA System Time. Once BTS2acquires the reverse link signal from the MS, BTS2 measures thedifference Tm 626 between BTS2 CDMA System Time 616 and the MS CDMASystem Time of the reverse link signal as measured at the receiver ofBTS2 614. The wireless network can then determine the timing error TE1622 of BTS1 CDMA System Time 402 according to the equation:${TE1} = {\frac{{Tm} - {\Delta \quad T}}{2} - {T1}}$

[0097] where TE1 622 is the timing error of BTS1 CDMA System Time 402,Tm 626 is the difference between BTS2 CDMA System Time 616 and the MSCDMA System Time of the reverse link signal as measured at the receiverof BTS2 614, ΔT is the time offset 608 between BTS1 CDMA System Time andBTS2 CDMA System Time measured at the MS, and T1 604 is the one-waypropagation delay from BTS1 to the MS. The wireless network uses the newΔTE to update the Neighbor List messages subsequently transmitted byBTS1 and BTS2.

[0098]FIG. 7 is a flowchart showing the steps of an exemplary method ofestablishing handoff of a mobile station from an asynchronous servingbase station (BTS1) to a synchronous target base station (BTS2). At step700, BTS1 sends a Neighbor List to the MS. The Neighbor List includesforward link characteristics for each neighbor base station such as anestimated pilot offset and the expected accuracy of the estimated pilotoffset.

[0099] At step 702, BTS1 measures RTD1 (606 in FIG. 4) to determine T1(604 in FIG. 6). At step 704, the MS evaluates the expected accuracy ofthe BTS2 pilot offset based on information received in the NeighborList. If the PN offset of BTS2 is accurately known, then, at step 712,the MS searches for the pilot signal of BTS2 within an estimated searchwindow. The width of the estimated search window is based on theinaccuracy of the PN offset estimate. After acquiring one pilot signalfrom a synchronous base station at step 712, the MS has an accurateestimate of CDMA System Time for any synchronous base station. At step714, the MS uses this synchronous base station timing information toefficiently search for the pilot signals of any other synchronous basestations in the Neighbor List.

[0100] If the MS cannot accurately determine the pilot offset of BTS2,then at step 706, the MS searches for the BTS2 pilot over the entirepilot code search range of 26.67 milliseconds. At step 708, the MSevaluates whether the pilot signals acquired can be unambiguouslyidentified. If the PN offsets of the pilot signals are spaced far apartcompared to the inaccuracy of the pilot offset estimates in the NeighborList, then the pilot signals may be uniquely identified after the fullPN search at step 706. If the PN offsets of the acquired pilot signalscannot be uniquely identified, or if the CDMA System Time of asynchronous base station cannot be determined, then at step 710, the MSreads CDMA system time from the sync channel of a synchronous basestation.

[0101] After performing step 710 or 714, or if BTS2 CDMA System Time isunambiguously determined at step 708, then at step 716 the MS measuresthe timing error of BTS2 in relation to BTS1. At step 718, the MS sendsinformation indicating the BTS2 timing error within a Pilot StrengthMeasurement Message (PSMM) to serving base station BTS1. The wirelessnetwork uses this timing error information to update information aboutthe timing error between BTS1 and BTS2 sent to mobile stations insubsequent Neighbor List messages.

[0102] In an exemplary embodiment, at step 720, BTS2 begins transmittingforward link frames to the MS at an offset TO2 (620 in FIG. 6). TO2 (620in FIG. 6) is calculated as described above. In an alternate embodiment,BTS2 does not begin transmitting forward link frames to the MS untilafter step 722, in which BTS1 adjusts its frame offset value from TO0(618 in FIG. 6) to TO′ (624 in FIG. 6) as described above. At step 724,BTS2 searches for the reverse link signal of the MS over an estimatedsearch window. The width of the estimated search window is based on thepath delay uncertainty. The location of the estimated search window isbased on the measured RTD1 (606 in FIG. 6).

[0103] At step 726, BTS2 acquires the reverse link signal transmitted bythe MS. Then, at step 728, BTS2 adjusts (deskews) its reverse linkdemodulation timing according to the actual arrival time of reverse linksignals from the MS.

[0104]FIG. 8 is a diagram of an advantageous “short sync channel” signalstructure that allows faster determination of CDMA System Time from asynchronous base station. Three separate forward link channels areshown, a pilot channel 800, a sync channel 802, and an identification(ID) channel 804. Pilot channel 800 may be a conventional pilot channelaccording to IS-95. Pilot channel 800 is transmitted by synchronous basestations at different PN offsets from the system time zero reference.For convenience of illustration, pilot channel 800 may be broken downinto a continuous repetition of frames 800A, 800B, 800C. Sync channel802 may be a conventional synchronization channel according to IS95.Sync channel 802 is transmitted with the beginning of a sync channelframe being time-aligned with the pilot channel 800, according to thesame PN offset. For convenience of illustration, sync channel 802 may bebroken down into a continuous repetition of frames 802A, 802B, 802C.

[0105] ID channel 804 is also shown in FIG. 3 as being time-aligned withthe PN offset of the pilot channel 800. However, ID channel 804 is not aconventional IS-95 channel. Advantageously, ID channel 804 istransmitted by synchronous base stations in addition to the conventionalpilot channel 800 and conventional sync channel 802. ID channel 804 isalso advantageously covered with a different Walsh code than pilotchannel 800 or sync channel 802 in order to maintain orthogonality ofthe forward channels. However, it is understood that the ID channel 804does not necessarily need to be orthogonal to the remaining overheadchannels. For convenience of illustration, ID channel 804 may be brokendown into a continuous repetition of frames 804A, 804B, 804C which arealigned in time with the PN sequence of the pilot channel 800.

[0106] The ID channel 804 is used to improve handoffs from asynchronousbase stations to synchronous base stations. In particular, the IDchannel 804 aids a mobile station in determining the pilot PN offset,and therefore the identity and timing of the target synchronous basestation to which it is handing off. As described above, the conventionalsync channel 802 contains a large amount of information in addition tothe pilot PN offset of the transmitting base station. None of thisadditional information is as immediately critical to the mobile stationduring handoff from an asynchronous base station to a synchronous basestation as the pilot PN offset. Thus, in the embodiment of FIG. 3, IDchannel 804 contains as little information as necessary for the basestation to rapidly determine the pilot PN offset of the transmittingbase station. For example, in one embodiment, the ID channel 804contains at least the 9-bit pilot PN offset. The ID channel 804 may alsocontain the 2-bit phase of the pilot PN sequence in 80 ms, and also a1-bit even/odd indication of the 80 ms epoch of the pilot PN sequence.These additional fields allow the mobile station to determine the PNlong code state. With this information, the mobile station can derivesystem time and synchronize to the transmitting base station's timing.

[0107] In the exemplary embodiment, the ID channel 804 is Golay encoded.For example, in the embodiment that has 12 bits (9 pilot PN offset, plus2 phase, plus 1 even/odd), the ID channel 804 uses a 24 bit (24, 12)Golay code word. Golay codes are well known in the art as being robust,efficient error correcting codes and will not be discussed in detailherein. Example Golay coding and decoding techniques are given in thebook “Error Control Coding: Fundamentals and Applications”, by Shu Linand Daniel J. Costello, Jr., ISBN 0-13-283796-X. However, othererror-correcting coding techniques such as convolutional encoding orother techniques well known in the art may be used for the ID channel804 without departing from the present invention.

[0108] Each ID channel frame 804A, 804B, 804C may contain one or morecode words with each code word containing the pilot PN offset, andoptionally the other information described above. The ID channel 804advantageously is continuously repeated. The continuous repetitionallows a mobile station that did not collect sufficient energy duringthe duration of a single code word to combine energy from multipleconsecutive code words in order to decode the code word and recover thepilot PN offset. In one embodiment, only those synchronous base stationsthat have at least one asynchronous base station as a neighbor wouldtransmit the ID channel 804.

[0109] In an alternate embodiment, the advantageous short sync channelstructure described above is used in synchronous base stations tofacilitate handoff from asynchronous to synchronous base stations. Forexample, in FIG. 7, the MS at step 710 reads CDMA System Time using theshort sync channel techniques described above.

[0110] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

We claim:
 1. A method for establishing a handoff of a wireless mobilestation from a serving base station to an asynchronous target basestation comprising: sending, from the serving base station to the mobilestation a neighbor list message indicative of an estimated timing errorof the asynchronous target base station; receiving from the mobilestation a measurement message indicative of a measured timing error ofthe asynchronous target base station; and at the serving base station,updating the estimated timing error based on the measured timing error.2. The method of claim 1 further comprising: after said updating,sending from the serving base station to a second mobile station aneighbor list message indicative of the estimated timing error.
 3. Themethod of claim 1 wherein the measurement message is a pilot strengthmeasurement message.
 4. The method of claim 1 further comprising thestep of determining a forward frame number from a forward link signaltransmitted by the asynchronous target base station.
 5. A method forestablishing a handoff of a wireless mobile station from an asynchronousserving base station to a synchronous target base station comprising:sending, from the asynchronous serving base station to the mobilestation a neighbor list message indicative of a pseudonumber (PN) offsetof a pilot associated with the synchronous target base station, whereinthe PN offset is based on an estimated timing error existing between theasynchronous serving base station and the synchronous target basestation; and receiving from the mobile station a measurement messageindicative of a measured timing difference between the PN offset and apilot signal received at the mobile station from the synchronous targetbase station.
 6. The method of claim 4 further comprising, at theasynchronous serving base station, updating the estimated timing errorbased on the measured timing difference.
 7. A wireless communicationsystem comprising: a serving base station for transmitting a neighborlist message indicative of an estimated timing error of an asynchronoustarget base station, receiving a measurement message indicative of ameasured timing error of the asynchronous target base station, andupdating the estimated timing error based on the measured timing error;and a wireless mobile station for receiving the neighbor list message,estimating a search window for use in searching for an asynchronouspilot code transmitted from the asynchronous target base station basedon said timing error, searching for the asynchronous pilot code withinsaid search window, measuring the received timing of the asynchronouspilot code, and sending the measurement message based on the measuredreceived timing.
 8. The wireless communication system of claim 6 whereinsaid serving base station is an asynchronous base station.
 9. Thewireless communication system of claim 1 wherein said serving basestation is a synchronous base station.
 10. The wireless communicationsystem of claim 1 wherein said serving base station comprises a globalpositioning satellite (GPS) receiver.
 11. A wireless mobile stationapparatus comprising: means for evaluating a timing error associatedwith the asynchronous target base station; means for estimating a searchwindow for use in searching for an asynchronous pilot code based on saidtiming error; and means for searching for the asynchronous pilot codewithin said search window.
 12. A wireless mobile station apparatuscomprising: means for evaluating a timing error associated with theasynchronous serving base station and the synchronous target basestation; means for estimating a search window for use in searching for asynchronous pilot pseudonumber (PN) signal based on said timing error;and means for searching for the pilot PN signal within said searchwindow.
 13. A wireless network base station apparatus comprising: meansfor transmitting to a mobile station a neighbor list message indicativeof an estimated timing error of an asynchronous target base station;means for receiving from the mobile station a measurement messageindicative of a measured timing error of the asynchronous target basestation; and means for updating the estimated timing error based on themeasured timing error.
 14. In a wireless network, an asynchronous basestation apparatus comprising: means for transmitting to a mobile stationa neighbor list message indicative of a pseudonumber (PN) offset of apilot associated with a synchronous target base station, wherein the PNoffset is based on an estimated timing error existing between theasynchronous base station and the synchronous target base station; andmeans for receiving from the mobile station a measurement messageindicative of a measured timing difference between the PN offset and apilot signal received at the mobile station from the synchronous targetbase station.